Foundation or pier drilling machines (also known in the industry as bored piling machines) are used to drill holes for the foundations of structures, such as buildings and bridges. The holes drilled may be, for example, up to 12 feet in diameter and up to an average of 120 feet deep. Once the hole is drilled, concrete is poured in to form a pier or footing.
The drilling tool of choice is an auger bit. The auger has helical, spiraling flanges or flights.
In typical drilling operations with an auger bit, the auger is placed onto the desired location in the ground and then rotated. Rotation of the auger occurs by rotating a kelly bar that is inserted into the auger. The kelly is coupled to a stationary mast by way of a rotary drive. The rotary drive rotates the kelly and allows the kelly to move vertically up and down.
In any drilling operation, the process of drilling creates spoil, or cuttings, that must be removed from the hole. In some drilling operations, fluid is circulated in and out to carry the spoil out of the hole. However, in many foundation or pier drilling operations, the spoil is simply lifted out of the hole by way of the auger bit.
The spoil collects on the flights of the auger during drilling. To remove the spoil, the auger is lifted out of the hole and swung over to one side of the hole. The auger bit is rotated and is in fact accelerated to a higher speed than normal drilling speed. This acceleration and high rotational speed flings or discharges the spoil from the flights of the auger.
In the prior art, drilling machines have had difficulty in transitioning the auger bit from drilling speeds to the higher spoil discharge speeds. When drilling, the operator rotates the auger bit at drilling speeds, which speeds are relatively low, using one set of drive gears. This arrangement allows the bit to be turned with high torques. To discharge spoil from the bit, the operator withdraws the bit from the hole and stops rotating the auger bit in order to engage another set of drive gears. This other set of drive gears rotates the bit at high speeds and low torques. Unfortunately, without high torque, bringing the bit, loaded with spoil, from zero to high speeds is time consuming and sometimes ineffective. It is analogous to a car completely stopping and then trying to move forward in a high (4th or 5th) gear. The bit initially rotates very slowly and gradually picks up speed. The slow spin-up time and acceleration of the spoil-laden auger bit adds delay and loss of productivity to overall drilling operations. The required stopping and/or shifting, if not done with careful operator control, is also a potential cause of damage to the prior art rotary drives.